One thing that i WAS going to suggest, is an indicator on the pathogen selection screen as to which levels and difficulties had been completed. Then I noticed you already had that. However, I only finally noticed it after completing around 15 levels!

Maybe it's just me, but the 3 little white stripes in the upper right corners were totally invisible to my brain.

I think at least changing the colors of those stripes to ie, bronze, silver, and gold, would really help them stand out and be noticed (for people like me). As well as put more visual emphasis on your achievements. Maybe have them animate onto the screen when you next open the pathogen selection page?

I would also like to see mutation icons that I select be highlighted more visibly while selected. I often hold my phone with my left hand, tap with my right. So my right hand covers the right side of my screen where the description is and noticeably changes as you select different mutations. Under my finger its hard sometimes to tell what I have selected so I have to look at the description to know for sure.

Lastly, I also feel the game is rather anticlimactic when you win a level. You know when it's been a good battle, but you know you've got them. The world has turned red, you watch the tickers finish ticking, there's a pause, and then you go to a 2 tone static victory screen where you can view your graphs. There's no victory sound, or animation, cut scene, or anything exciting. It would be nice to have something more there. After all, you did just kill everybody in the world! That's a pretty big deal!